(a) Field of the Invention
The present invention relates to a switching mode power supply (SMPS). More specifically, the present invention relates to a switching mode power supply for reducing power consumption in the standby operation mode.
(b) Description of the Related Art
In general, a switching mode power supply (SMPS) converts a direct current (DC) supply voltage into one or more DC output voltages that have a greater or lesser magnitude than the DC supply voltage. SMPSs are widely used in power electronic devices, particularly battery-powered devices such as portable cellular phones and laptop computers. Typically, SMPSs have a normal operation mode and a standby operation mode. Power electronic devices consume a large amount of power in the normal operation mode, as compared to the standby operation mode. Power electronic devices may automatically enter the standby operation mode when a user does not use the device for a predetermined time frame, and may automatically enter the normal operation mode when the user resumes the use of the device.
In most electronic devices, power consumption in the standby operation mode is much less than power consumption in the normal operation mode. To reduce power consumption in the standby operation mode, input power to the electronic device is limited. Conventionally, to meet such control requirements, an output voltage of the SMPS is reduced, or an additional power source is used to reduce power consumption in the standby operation mode. However, such approaches for entering the standby operation mode are not desirable because they require additional components, thereby resulting in higher production costs. Moreover, the extent to which power consumption of the electronic device can be decreased is limited because it is likely that an output voltage below a predetermined level for operating the electronic devices may occur.
When using a conventional SMPS in the standby operation mode, a substantial switching loss occurs in the SMPS despite the reduction in power consumption due to a reduced output voltage. In addition, the conventional SMPS changes a duty cycle of a power switch in order to compensate for changes in power requirements at its output end, and operate at a predetermined frequency regardless of the amount of supplied power. As a result, the power switch within the conventional SMPS operating in the standby operation mode performs switch-on/off operations at the same rate as when the SMPS is operating in the normal operation mode. In the standby operation mode, such switching operations consume a considerable amount of power, which prevents greater decreases in power consumption in the standby operation mode.
In an attempt to solve such problems, a SMPS has been proposed which uses active circuitry to provide the normal operation mode, and a low power burst mode that enables a standby operation of an electronic device. U.S. Pat. No. 6,252,783 discloses the operation and configuration of such an SMPS. In the normal operation mode, the active circuitry couples an output voltage of the SMPS to a conventional switch driver circuit (or a control module circuit). This switch driver circuit changes a duty cycle of an output having a fixed frequency of a switch driver in order to regulate the output voltage of the SMPS to a desired level.
When the electronic device is operating in the low power burst mode, the active circuitry separates the output voltage of the SMPS from the switch driver circuit and applies a periodic signal to the switch driver. This periodic signal causes the switch driver to provide an output signal having a fixed frequency for predetermined time intervals. These time intervals are interleaved with time intervals during which the output of the switch driver is inactive, i.e., the switch driver turns the switch off. In addition, when the electronic device is operating in the low power burst mode, the active circuitry applies an input signal to the switch driver. This signal causes the switch driver to repeatedly turn the switch on and off at a fixed frequency, i.e., at a minimum duty cycle. The low power burst mode during which the switch driver provides the output having the fixed frequency is appropriately controlled so that a supply voltage to the switch driver may be varied between two reference voltages.
The above described SMPS having the low power burst mode reduces the switching loss by performing and stopping switch-on/off operations for a predetermined amount of time in the standby operation mode. This reduces power consumption. In addition, the SMPS can maintain a lower output voltage in the standby operation mode than in the normal operation mode, and can control the switch-on/off operations of the switch in the standby operation mode by using the low power burst mode at a predetermined time interval irrespective of the output voltage of the SMPS.
However, the SMPS having the low power burst mode may have audible noise as the maximum amplitude of current increases. As the maximum amplitude of current increases, so does the importance of the switching loss. In addition to the switching loss, the SMPS may also incur conduction loss and core loss. When a light load is used by the entire system, the switching loss increases relatively, and thus, frequent audible noise is caused and more power is consumed with the increase in the maximum amplitude of current.